1. Field of the Invention
The present invention relates to an encoder that detects a linear motion position of a body to be measured or a rotation angle of a body to be measured and includes an accuracy correction function.
2. Description of the Related Art
A machine tool includes a table that linearly moves a workpiece or the like and a motor that drives the table. The machine tool is equipped with an encoder that detects the linear motion position of the table or the rotation angle of the motor, for example, a linear scale and a rotary encoder.
A measuring body such as the linear scale or the rotary encoder periodically generates an analog signal in accordance with movement of a body to be measured such as the table or the motor. Then, correction data is calculated based on a deviation between position data obtained from the analog signal and a reference value (data indicating a true position). The correction data is generated and stored in a storage unit of a signal processing unit of the measuring body beforehand. The correction data includes both of periodic and aperiodic error components.
When the machine tool is actuated, the correction data is applied to position data obtained from the analog signal related to the body to be measured to correct the linear motion position or the rotation angle of the body to be measured. Japanese Laid-open Patent Publications No. 2006-170790, No. 2011-141247, No. 2006-234723 and No. 2007-64771 disclose encoders that correct position data in this way.
However, the amount of correction data calculated beforehand as described above is large and requires a large space for the storage unit of the signal processing unit. This has led to an increase in the chip size of a signal processing unit including the storage unit, resulting in an increase in the dimensions of the measuring body.
To cope with the problem, Japanese Laid-open Patent Publications No. 10-311741 and No. 2003-254785 describe use averaging or calculating a position index on the basis of time variations to automatically correct the analog signal when using an encoder.
However, the signal processing unit of the encoder described above corrects errors caused by an offset voltage, amplitude difference and phase difference of sinusoidal analog signal. Therefore, there is a problem that aperiodic error components generated at aperiodic interval in all positional data are unable to be corrected.
The present invention has been made in light of these circumstances and an object of the present invention is to provide an encoder capable of correcting aperiodic error components without increasing the capacity of a storage unit.